This invention relates to a a bistable electro-optic material display and to a method for addressing such a display. The term xe2x80x9celectro-optic materialxe2x80x9d is used herein in its conventional meaning in the art to refer to a material having first and second display states differing in at least one optical property, the material being capable of being changed from its first to its second display state by application of an electric field to the material. The terms xe2x80x9cbistablexe2x80x9d and xe2x80x9cbistabilityxe2x80x9d are used herein in their conventional meanings in the art to refer to displays comprising display elements having first and second display states differing in at least one optical property, and such that after any given element has been driven, by means of an addressing pulse of finite duration, to assume either its first or second display state, after the addressing pulse has terminated, that state will persist for at least several times the minimum duration of the addressing pulse required to change the state of the display element. These terms are also to be construed to include materials having more than two display states which are stable in this manner.
Electrophoretic and other electro-optic displays have been the subject of intense research and development for a number of years. Such displays can have attributes of good brightness and contrast, wide viewing angles, state bistability, and low power consumption when compared with liquid crystal displays. Nevertheless, problems with the long-term image quality of electrophoretic displays have prevented their widespread usage. For example, particles that make up electrophoretic displays tend to cluster and settle, resulting in inadequate service-life for these displays.
An encapsulated, electrophoretic display typically does not suffer from the clustering and settling failure mode of traditional electrophoretic devices and provides further advantages, such as the ability to print or coat the display on a wide variety of flexible and rigid substrates. (Use of the word xe2x80x9cprintingxe2x80x9d is intended to include all forms of printing and coating, including, but without limitation: pre-metered coatings such as patch die coating, slot or extrusion coating, slide or cascade coating, curtain coating; roll coating such as knife over roll coating, forward and reverse roll coating; gravure coating; dip coating; spray coating; meniscus coating; spin coating; brush coating; air knife coating; silk screen printing processes; electrostatic printing processes; thermal printing processes; ink jet printing processes; and other similar techniques.) Thus, the resulting display can be flexible. Further, because the display medium can be printed (using a variety of methods), the display itself can be made inexpensively.
One problem in addressing electrophoretic and similar electro-optic display materials is that typically such materials do not exhibit any significant threshold, i.e., even a small electric field will cause the material to change its display state if the small field is sustained for a considerable period of time. Consequently, it is usually not possible to address such materials using a passive matrix addressing scheme such as is often used with liquid crystal displays. Accordingly, such display materials are addressed using (a) a xe2x80x9cdirect-drivexe2x80x9d addressing scheme, in which each pixel is provided with a separate electrode which can be addressed via its own drive line; (b) an active matrix addressing scheme in which each pixel is provided with an associated non-linear element (such as a transistor); or (c) a moving head addressing scheme, in which a head containing a number of individually controllable discrete electrodes is moved across the display, so that each electrode writes one line of the display.
All three of these addressing schemes have disadvantages. A direct-drive addressing scheme is practicable for a textual display (see, for example, International Application PCT/US99/16652, Publication No. WO 00/05704, which describes such a display using 63 electrodes for each text character) but impracticable for a graphic display, since providing the large number of drive lines required for a graphic display would necessitate excessively complex and expensive circuitry. Active matrix addressing schemes require complex and relatively expensive circuitry, the cost of which typically far exceeds the cost of the electro-optic medium itself. Furthermore, since the cost of producing active matrix addressing circuitry is typically directly proportional to the area of the display, this type of addressing scheme is impracticable for very large displays, for example signs intended as electronic billboards in sports stadia or as message boards in large exhibition halls. Moving head addressing schemes tend to be inexpensive but, since the head needs to be in intimate contact with the electro-optic medium to produce the large electric fields needed to change the display state of most electro-optic materials, the friction between the moving head and the electro-optic material causes wear on the material and limits the life of the display. Furthermore, a moving head is highly sensitive to any slight unevenness in the display material, and in very large signs it is difficult to avoid such unevenness in the display material, especially if the large sign is to be used outdoors, where wind gusts might slightly distort the medium.
The present invention seeks to provide an electro-optic display, and a method for addressing such a display, which are comparable in cost and complexity to a moving head addressing scheme, but which reduce or eliminate frictional wear on the electro-optic material and which are less susceptible to unevenness of the electro-optic material.
Accordingly, this invention provides an electro-optic display comprising:
a bistable electro-optic material having on one side thereof a viewing surface on which the display can be viewed, and on the opposed side thereof a writing surface;
a deformable member disposed adjacent but spaced from the writing surface of the electro-optic material, the deformable member bearing a plurality of elongate electrodes formed on its surface facing the writing surface of the electro-optic material; and
a movable member disposed on the opposed side of the deformable member from the electro-optic material and movable relative to the deformable member in a direction substantially parallel to the length of the elongate electrodes, the movable member contacting and deforming the deformable member such that a portion of each elongate electrode contacts or lies closely adjacent the writing surface of the electro-optic material.
This invention also provides a method for addressing an electro-optic display of the invention as defined above. In this addressing method, the movable member is placed in a first position relative to the deformable member and a first set of potentials are placed on the elongate electrodes, thereby writing a first column of pixels of the display, each of this column of pixels being defined by the portion of one elongate electrode which contacts or lies closely adjacent the writing surface of the electro-optic material when the movable member is in its first position. The movable member is then moved to a second position relative to the deformable member, this second position being spaced from the first position, and a second set of potentials are placed on the elongate electrodes, at least one of the second set of potentials differing from the first set of potentials, thereby writing a second column of pixels of the display, each of this column of pixels being defined by the portion of one elongate electrode which contacts or lies closely adjacent the writing surface of the electro-optic material when the movable member is in its second position.